Regiospecific no-carrier-added radiobromination and radioiodination of aryltrimethyl group IVb organometallics

A series of para-substituted aryltrimethyl-silicon, -germanium, and -tin compounds were treated at room temperature with no-carrier-added (n.c.a.) ⁷⁷Br and ¹³¹I to compare their utility as substrates for regiospecific aromatic halogenation. Dichloramine-T was used as an in situ oxidant in acidic, polar, or non-polar reaction solvents. N.c.a. aromatic halodemetallation was regiospecific for all substrates, radiochemical yields were generally higher for ¹³¹I than ⁷⁷Br, and halogenation yields increased as Si < Ge < Sn. Arguments are presented for a substitution mechanism involving a σ-complex intermediate with bond formation as the yield-determining step. Although high yields were obtained for the n.c.a. aromatic halogenodestannylations irrespective of aromatic substituent or solvent, significant chlorination occurred as a side-reaction. N.c.a. aromatic halogenodegermylation produced high radiochemical yields with insignificant chlorination, but the radiohalogenation yields were sensitive to ring deactivation and solvent effects. N.c.a. aromatic halogenodesilylation gave high yields only in activated aromatic systems. The relative benefits of each demetallation reaction for regiospecific n.c.a. radiolabelling or preparative halogenation of aromatic rings are outlined.